The present invention relates to a circuit for communicating electronic signals. In particular, the present invention relates to a system for compensating for signal level variations in communications circuits.
It is often desirable in communications circuits to compensate for variations in power supply voltages. It is also desirable to compensate for variations in transistor characteristics arising from changes in operating conditions, such as temperature. These features are particularly advantageous for low voltage applications, where a variation in power supply voltage and/or base-emitter voltage can cause output signal voltages to vary sufficiently so as to no longer comply with the appropriate signal specification.
Attempts have been made at compensating for power supply voltage variations in communications circuits where the input signals to the communications circuits are referenced to the power supply voltage. For instance, in one implementation, a control circuit monitors the power supply voltage and injects compensating currents into the communication circuit input stage to counter the effect of power supply variations on the current levels in the input stage. However, such implementations are unsuitable for high frequency applications using low power supply voltages, since the injected current can cause the voltage drop across the current sink transistors coupled to the input stage to become undesirably low. Further, such implementations are ill-suited for applications where the power supply voltage is low or is susceptible to large variations, since the current sink transistors driving the input stage may become saturated. Also, voltage compensation at the input stage may leave insufficient head room for output signal swing.
Accordingly, there remains in need for a system for compensating for signal level variations in communications circuits arising from variations in supply voltage and/or transistor characteristics.
According to the present invention, there is provided a signal level compensating system for communications circuits which addresses the deficiencies of the prior art. The term xe2x80x9ccommunications circuitxe2x80x9d is used herein in its generic sense as any circuit which is capable of conveying an electronic signal between an input and an output.
According to an aspect of the present invention, the signal level compensator comprises a voltage-follower stage, a sensor and an output signal compensator. The voltage-follower stage includes a signal input for receiving an input signal, a signal output, and at least one transistor coupled between the signal input and the signal output for providing an output signal responsive to the input signal. The sensor senses variations in at least one of the power supply voltage and transistor characteristics of the transistor and provides a control signal indicative of the variation. The output signal compensator is coupled to the signal output and provides a compensator output signal responsive to the control signal for reducing the impact of the variations on the output signal.
According to a further aspect of the present invention, there is provided a signal-level shifter including: a voltage-follower stage including a voltage-follower input for receiving an input signal, a voltage-follower output, and a voltage-follower stage transistor coupled between the voltage-follower input and the voltage-follower output for providing an output signal at the voltage-follower output responsive to the input signal; an output stage coupled to the voltage-follower stage and including an output stage transistor for shifting a signal level of the output signal a sensor for sensing variation in at least one of characteristic of the transistors and voltage of a power supply powering the signal-level shifter and providing a control signal indicative of the variations; and an output signal compensator control signal for reducing the impact of the variations an the output signal.
According to a further aspect of the present invention, there is provided a method of compensating for signal-level variations in communications circuits. The method includes the steps of: providing a voltage-follower including a signal input for receiving an input signal, a signal output, and at least one transistor coupled between the signal input and the signal output for providing an output signal at the signal output responsive to the input signal; sensing variations in at least one of characteristics of the transistor and a voltage of a power supply powering the voltage-follower and providing a control signal indicative of the variations; and adding a compensating signal to the output signal, the compensating signal being responsive to the control signal and having a phase opposite to the variations.
In one aspect of the invention, the voltage-follower stage comprises a pair of transistors configured to receive differential input signals, and a resistive load stage coupled to the outputs of the voltage-follower transistors. The sensor comprises a transistor matched to the voltage-follower transistors and a resistor matched to the resistors of the resistive load stage, and provides control signals to the output signal compensator representative of variations in power supply voltage and voltage drop (eg. base-emitter, gate-source) of the voltage-follower transistors. The output signal compensator is coupled to the sensor, and comprises transistors configured as a current mirror coupled to the resistive load stage.
The output current level provided by the output signal compensator is responsive to the control signal from the current sensor, and cancels variations in output signal at the resistive load stage due to variations in power supply voltage and voltage drop at the voltage-follower stage.
In another aspect of the invention, the output of the voltage-follower stage is coupled to a pair of series-coupled differential voltage-followers for outputting the input signals in accordance with a desired signal specification. The sensor comprises transistors matched to the transistors of the voltage-follower stage and the series-coupled voltage-followers, and provides control signals to the output signal compensator representative of variations in power supply voltage and voltage drop (eg. base-emitter, gate-source) of the transistors of the voltage-follower stage and the series-coupled voltage-followers. The output signal compensator is coupled to the sensor and provides an output current responsive to the control signal from the current sensor, which cancels variations in output signal voltage at the output of the last of the series-coupled differential voltage-followers due to variations in power supply voltage and voltage drop of the transistors of the voltage-follower stage and the series-coupled voltage-followers.
In a preferred embodiment of the invention, the signal level compensating system is implemented as an integrated circuit on a single substrate, is powered by 3 volt power source, and receives CML input signals from an emitter-follower output stage external to the integrated circuit. The voltage-follower stage comprises a pair of NPN transistors, and the current mirror comprises insulated nMOS transistors for allowing the current mirror to operate below substrate voltage. The output of the voltage-follower stage is coupled to a pair of series-coupled emitter-followers for outputting the input signals as ECL output signals. The voltage-follower stage is coupled at its input to a signal-conditioner input stage which comprises a first differential buffer, a differential voltage-follower coupled to the output of the differential buffer, and a second differential buffer coupled to the output of the differential-emitter follower of the signal-conditioner input stage. The signal-conditioner input stage boosts the signal level of the input signals to the voltage-follower stage to provide sufficient headroom for compensating the power supply and base-emitter voltage variations.